Step one: Use an iron maiden on the "dead" immortal witch's body so that the spikes make it so that she instantly dies when her curse and magic resuscitate her.
Step two: Dig a very, VERY deep hole.
Step three: Throw in the bitch witch.
Step four: Alternate between dirt and rocks when filling the hole.
Step five: Press the ground to further compact it and make it even harder, cover with A LOT more rocks.
Step six: Find a way to carry a HUGE boulder and place it on top of the rocks.
Step seven: Ask your witch friend to curse the boulder so those that touch it will regret it and cover the boulder with ominous signs.
Step eight: Spread news in human cities about a cursed boulder that will curse to death those that touch it.
There, she won't come back for the next few hundred to a few thousand years.